1. Field of the Invention
The present invention relates to a navigation device and method therefor which displays a recommended route which can be accomplished with reference to the shortest timeframe, when searching a route from a present position to a destination.
2. Description of the Related Art
Navigation devices for use in a moving body, for example an automobile, have recently enjoyed increasing popularity. Such devices calculate a recommended route from a present position of the vehicle to a destination on the basis of a destination set by a user. The recommended route is then displayed. Some devices provide for indicating the direction of turning with voice commands so that the route may be correctly selected at the next intersection.
Devices which have the function of searching for recommended routes entail the problem of searching for the route on the basis of some kind of standard. Normally this is defined as a route which is "short" with respect to distance or time. However the selection of the shortest route with respect to time entails the use of expressways and the resultant problem of increases in tolls. It is difficult to search for a route in consideration of all parameters. Normally a method of selecting a route, which satisfies a user, is adopted in which the user selects each condition in order of priority.
The present invention proposes an improvement related to a device which searches a route on the basis that travelling time is prioritized. When applied to an automobile, the present invention prioritizes time by analyzing the speed limits set on each road divided into a number of sections. The recommended route is assumed to be the combination of routes having the smallest total for the time required for the divided sections.
However if there is an obstacle such as a railway crossing along such a route which delays the passage of the moving body, the route as calculated above may not be the shortest route as regards time and a detour may be shorter.
An invention which addresses the above problem is disclosed for example in JP-A-9-5100. In this invention, the presence of railway crossings and the timetables of main stations are included in road data. When a route is searched on roads which have railway crossings, routes which include railway crossings with long waiting times are excluded by a weighting process which corresponds to the number of trains passing in unit time.
In this way, the effect of waiting time at railway crossings may be reduced and a destination may be reached generally within a fixed time. However the problem has arisen that the reliability of recommended routes displayed is low due to the fact that it is not possible to determine whether or not a given railway crossing will actually delay the passage of the moving body.
JP-A-7-229746 discloses a device in which the timetables of all modes of transportation are stored. This enables an estimation of the waiting time at railway crossings which intersect with the road by displaying superimposed timetables on the road map in conjunction with the progress of the moving body.
However the above device simply displays timetables and thus the driver or passenger must determine whether it would be quicker to wait at the railway crossing or to make a detour. Such decisions are difficult for a person riding in a moving body and in particular safety considerations arise by a driver being distracted by such decisions.
Since conventional navigation devices are constructed as above, it is possible to select a route on the basis of the presence of an obstruction such as a railway crossing. However the probability that the recommended route is the shortest route is low due to the fact that displayed recommended routes do not consider the amount of time for which the vehicle may actually be delayed.